1. Field of the Invention
The present invention relates to a technique wherein a plurality of mobile stations access a base station randomly for performing packet transmission in a cellular system which uses radio waves of the same frequency repetitively.
Further, the present invention relates to a timeslot assignment method in a TDD system for mobile communications, a base station and a mobile station using the method.
2. Description of the Related Art
It is well known that random access controlled by a base station is necessary for preventing throughput reduction caused by collisions of packets which are sent simultaneously from a plurality of mobile stations when the mobile stations access a base station randomly.
As a conventional random access method, an ICMA-PE (Idle-Signal Casting Multiple Access with Partial Echo) method is proposed in Japanese patent application No.1-240822. In the ICMA-PE method, a mobile station sends a signal after verifying that a timeslot is idle by checking a downlink notification signal.
More specifically, in the wireless communication method disclosed in the Japanese patent application No.1-240822, a collision control field is added after a downlink information unit. The collision control field includes an Idle/Busy (I/B) bit, a Receive/Non-receive bit (R/N) and Partial Echo bits (PE). In addition, length information W is added before an uplink information unit. A mobile station sends the length information to a base station in an uplink communication. According to the length information W, the base station decides a number of units where I/B is presented. Other mobile station accesses a timeslot where I is indicated in the I/B field.
The above-mentioned conventional packet transmission method has defects in that adaptability to a cellular system where the same frequency waves are used repeatedly is low and that high throughput can not be obtained.
That is, in the cellular system, there are timeslots having low interference and timeslots having high interference according to a position or a transmission power of a mobile station which uses the same frequency in another cell. Thus, when a mobile station which is far from the base station accesses a timeslot having high interference among a plurality of available timeslots, desired CIR (Carrier-to-Interference Ratio) can not be obtained since receive level at the base station is lowered due to propagation loss. Therefore, transmission failure occurs and high throughput can not be obtained. In the above description, CIR is a ratio of a desired wave (carrier) power to an interference wave power. The larger the CIR is, the smaller interference to the desired wave is. Therefore, probability of transmission failure becomes small when CIR is large since interference amount of the desired wave becomes small.
In addition, when a plurality of mobile stations tries to send data, a collision occurs when the mobile stations tries to send data simultaneously by a timeslot which is detected first. Thus, the possibility that the base station does not receive the data becomes high and throughput reduces.
Further, since the possibility that an idle timeslot having high interference is used in an adjacent cell is high, when a mobile station located in the vicinity of the periphery of a cell accesses an idle timeslot having high interference, it provides high interference to a communication in the adjacent cell in addition that communication failure ratio becomes high.
These problems occur because selection of idle timeslots is not performed in the above-mentioned packet transmission method.
In another aspect of the related art, conventionally, in a TDD (Time Division Duplex) system wherein carriers are assigned to timeslots for communication, uplink and downlink timeslot configuration is symmetric. The reason is that uplink and downlink traffic is almost symmetric in voice communication.
As multimedia services become widespread, non-voice traffic is increasing. Thus, it is conceivable that various information providing services and applications for e-mail, information sending from users and the like will be used more and more in the future. From the viewpoint of traffic of the applications and services, since data distribution from a database may be main traffic in the information providing services, it can be considered that downlink traffic becomes large in the network. In the uplink direction, information traffic from users will increase. Therefore, in non-voice communication such as data and images, there will be many cases where uplink traffic and downlink traffic are asymmetrical.
In such cases, when symmetrical timeslot assignment is used in the TDD system in the same way as conventional voice communication, a following problem arises. If timeslots are provided in accordance with communication of a direction which sends larger traffic, timeslots for communication of another direction remain unused. If timeslots are provided in accordance with communication of a direction which sends smaller traffic, timeslots for communication of another direction is insufficient. As a result, information transmission having high serviceability and high efficiency can not be provided.
For overcoming such problems, in Lan Chen, Susumu Yoshida, Hidekazu Murata and Shouichi Hirose, “A dynamic timeslot assignment algorithm for asymmetric traffic in multimedia TDMA/TDD mobile radio”, IEICE Trans. Fundamentals, vol. E81—A, pp. 1358–1366, no. 7, July 1998, a timeslot assignment method for asymmetric traffic is disclosed. In the conventional timeslot assignment method shown in this document, as shown in FIG. 1, one uplink/downlink timeslot switching boundary (TDD boundary) is provided in a frame. According to uplink and downlink traffic, the TDD boundary is moved within movable bounds. As shown in FIG. 1, although timeslots 4 and 5 are available, they can not be used as uplink. Thus, timeslots can not be used efficiently by this method.
In addition, control for accepting packet transmission is performed in the following according to the method in the above-mentioned document. When a mobile station sends a request timeslot number, which is a number of timeslots per one frame to be used for data transmission, to a base station, the base station tries to assign timeslots for the mobile station. At this time, when available timeslots is insufficient even if the TDD boundary is moved, the base station refuses data transmission. In addition, even when the number of timeslots which exist is larger than a number which is requested by the mobile station, a number of assigned timeslots is the same as the number which is requested. Further, when new available timeslots appear in data transmission and timeslots more than requested timeslots becomes available, the number of assigned timeslots is the same as the number which is requested.
As mentioned above, in the conventional timeslot assignment method, the TDD boundary is moved for adapting to asymmetrical uplink and downlink traffic. However, since there is only one boundary between uplink and downlink timeslots, when timeslots adjacent to the boundary are used, it is impossible to assign a downlink timeslot to an uplink timeslot area and to assign an uplink timeslot to a downlink timeslot area even when timeslots other than the timeslots adjacent to the boundary are released. Therefore, the released timeslots can not be utilized. Thus, the utilization of timeslot resources can not be maximized. As a result, frequency utilization efficiency is low, transmission delay becomes large, and in data transmission, transmission incompletion ratio becomes high.
In addition, when the number of idle timeslots is smaller than the number requested by a mobile station in the case of traffic congestion, the data transmission request is denied, and data is discarded or the mobile station wait for retransmission. Thus, data transmission incompletion ratio becomes high. In addition, transmission delay becomes large.
When the number of idle timeslot is larger than the number of requested timeslots during off-peak times, or, when new available timeslots appear during communication, it is possible to perform higher speed data transmission if the mobile station and the base station have means for data transmission using timeslots more than requested number or currently used timeslots. However, in the conventional technique, idle timeslots are not used actively. Thus, there is a problem that throughput is low.